Bio-oils of varying properties and compositions are obtained using numerous methods and processes. Bio-oils may be obtained for example from biomass using any suitable thermal treatment, pyrolysis and the like.
Pyrolysis is generally understood as the chemical decomposition of organic materials by heating in the absence or with limited supply of oxidizing agent such as air or oxygen. Pyrolysis can be used for converting biomass to pyrolysis oil which is an example of bio-oil. Commercial pyrolysis applications are typically either focused on the production of charcoal (slow pyrolysis) or production of liquid products (fast pyrolysis), the pyrolysis oil. Both the slow pyrolysis and the fast pyrolysis processes may be used for the manufacture of pyrolysis oil.
During pyrolysis of biomass, for example of lignocellulosic material, carried out typically at temperatures in the range 400-700° C., most of the cellulose and hemicellulose and part of lignin typically disintegrate to form smaller and lighter molecules which are vapors at the pyrolysis temperatures. During cooling some of the vapors condense forming a liquid product, called pyrolysis oil.
Bio-oils are complex mixtures of chemical compounds, including reactive aldehydes and ketones. Said reactive compounds react with each other whereby complex molecules having higher molecular weight are formed and the viscosity of bio-oil is increased. For example biomass derived pyrolysis oil typically comprises water, light volatiles and non-volatiles. Further, pyrolysis oil has high acidity, which typically leads to corrosion problems, substantial water content, and high oxygen content.
Wood-based pyrolysis oil is the product of pyrolysis of wood or forest residues and it contains typically carboxylic acids, aldehydes, ketones, carbohydrates, thermally degraded lignin, water, and alkali metals. The oxygen-containing compounds (typically 40-50 wt-%) and water (typically 15-30 wt-%) make pyrolysis oils chemically and physically unstable. Although pyrolysis oils have higher energy density than wood, they are acidic (pH˜2) and incompatible with conventional fuels. Furthermore pyrolysis oils have high viscosity and high solid content. Poor stability and high acidity are one of the key problems in utilizing the pyrolysis oil or storing them for longer periods.
Due to its instability bio-oil is rapidly transformed to semisolid and gradually solid material, which is difficult to store or use for any further purposes. Thus, according to present practice it is necessary to process the bio-oils rapidly further in order to avoid the problems relating to stability.
Despite the ongoing research and development relating to bio-oils, there is still a need to provide a process for modifying bio-oil for obtaining bio-oils with increased stability.